Although the transportation sector is mainly fueled by petroleum, the amount of natural gas used for vehicle fuel has grown to 43.4 billion cubic feet (bcf) in 2017. From 1990 to 2017 the average annual rate of growth was 20.7%.

According to the Energy Information Administration’s Annual Energy Outlook, more than half (52%) of natural gas vehicle fuel was used by heavy trucks in 2017. Buses used 27% and light-duty vehicles, typically in fleets, accounted for 20% of the vehicle fuel used. Locomotives used only 1%.

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Note that it takes about 110 scf of NG to replace the energy of 1 gallon of gasoline, and around 135 to replace the energy of 1 gallon of diesel. On top of this is the low flame speed of methane, which results in the fuel burning later in the piston's descent and consequent lower conversion of energy to work.

The upshot of this is that 43.4 bcf of natural gas displaces less than 400 million gallons of liquid petroleum fuels. This is a drop in the bucket of US consumption.

At one point recently the USA was on track to have NG dispensing at many chains of truck stops, which would displace diesel. The current (temporarily) low price of crude oil has apparently interrupted this changeover. The curve is still climbing but nowhere near as rapidly as it could have; the trucking industry could easily consume a trillion cubic feet per year.